add clearance support for grid-based pathfinding

[titoasty]

Clearance is the way to deal with agents bigger than one grid cell.

Here is a good article, in case of : https://harablog.wordpress.com/2009/01/29/clearance-based-pathfinding/

[xaguzman]

Never heard about that, seems pretty cool and useful!
Should totally add it in the future...lack of time